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Encode::Unicode(3pm) Perl Programmers Reference Guide Encode::Unicode(3pm)
NAME
Encode::Unicode -- Various Unicode Transformation Formats
SYNOPSIS
use Encode qw/encode decode/;
$ucs2 = encode("UCS-2BE", $utf8);
$utf8 = decode("UCS-2BE", $ucs2);
ABSTRACT
This module implements all Character Encoding Schemes of Unicode that are offi-
cially documented by Unicode Consortium (except, of course, for UTF-8, which is a
native format in perl).
<http://www.unicode.org/glossary/> says:
Character Encoding Scheme A character encoding form plus byte serialization.
There are Seven character encoding schemes in Unicode: UTF-8, UTF-16, UTF-16BE,
UTF-16LE, UTF-32 (UCS-4), UTF-32BE (UCS-4BE) and UTF-32LE (UCS-4LE), and UTF-7.
Since UTF-7 is a 7-bit (re)encoded version of UTF-16BE, It is not part of Uni-
code’s Character Encoding Scheme. It is separately implemented in Encode::Uni-
code::UTF7. For details see Encode::Unicode::UTF7.
Quick Reference
Decodes from ord(N) Encodes chr(N) to...
octet/char BOM S.P d800-dfff ord > 0xffff \x{1abcd} ==
---------------+-----------------+------------------------------
UCS-2BE 2 N N is bogus Not Available
UCS-2LE 2 N N bogus Not Available
UTF-16 2/4 Y Y is S.P S.P BE/LE
UTF-16BE 2/4 N Y S.P S.P 0xd82a,0xdfcd
UTF-16LE 2 N Y S.P S.P 0x2ad8,0xcddf
UTF-32 4 Y - is bogus As is BE/LE
UTF-32BE 4 N - bogus As is 0x0001abcd
UTF-32LE 4 N - bogus As is 0xcdab0100
UTF-8 1-4 - - bogus >= 4 octets \xf0\x9a\af\8d
---------------+-----------------+------------------------------
Size, Endianness, and BOM
You can categorize these CES by 3 criteria: size of each character, endianness,
and Byte Order Mark.
by size
UCS-2 is a fixed-length encoding with each character taking 16 bits. It does not
support surrogate pairs. When a surrogate pair is encountered during decode(), its
place is filled with \x{FFFD} if CHECK is 0, or the routine croaks if CHECK is 1.
When a character whose ord value is larger than 0xFFFF is encountered, its place is
filled with \x{FFFD} if CHECK is 0, or the routine croaks if CHECK is 1.
UTF-16 is almost the same as UCS-2 but it supports surrogate pairs. When it
encounters a high surrogate (0xD800-0xDBFF), it fetches the following low surrogate
(0xDC00-0xDFFF) and "desurrogate"s them to form a character. Bogus surrogates
result in death. When \x{10000} or above is encountered during encode(), it
"ensurrogate"s them and pushes the surrogate pair to the output stream.
UTF-32 (UCS-4) is a fixed-length encoding with each character taking 32 bits.
Since it is 32-bit, there is no need for surrogate pairs.
by endianness
The first (and now failed) goal of Unicode was to map all character repertoires
into a fixed-length integer so that programmers are happy. Since each character is
either a short or long in C, you have to pay attention to the endianness of each
platform when you pass data to one another.
Anything marked as BE is Big Endian (or network byte order) and LE is Little Endian
(aka VAX byte order). For anything not marked either BE or LE, a character called
Byte Order Mark (BOM) indicating the endianness is prepended to the string.
CAVEAT: Though BOM in utf8 (\xEF\xBB\xBF) is valid, it is meaningless and as of
this writing Encode suite just leave it as is (\x{FeFF}).
BOM as integer when fetched in network byte order
16 32 bits/char
-------------------------
BE 0xFeFF 0x0000FeFF
LE 0xFFeF 0xFFFe0000
-------------------------
This modules handles the BOM as follows.
· When BE or LE is explicitly stated as the name of encoding, BOM is simply
treated as a normal character (ZERO WIDTH NO-BREAK SPACE).
· When BE or LE is omitted during decode(), it checks if BOM is at the beginning
of the string; if one is found, the endianness is set to what the BOM says. If
no BOM is found, the routine dies.
· When BE or LE is omitted during encode(), it returns a BE-encoded string with
BOM prepended. So when you want to encode a whole text file, make sure you
encode() the whole text at once, not line by line or each line, not file, will
have a BOM prepended.
· "UCS-2" is an exception. Unlike others, this is an alias of UCS-2BE. UCS-2 is
already registered by IANA and others that way.
Surrogate Pairs
To say the least, surrogate pairs were the biggest mistake of the Unicode Consor-
tium. But according to the late Douglas Adams in The Hitchhiker’s Guide to the
Galaxy Trilogy, "In the beginning the Universe was created. This has made a lot of
people very angry and been widely regarded as a bad move". Their mistake was not
of this magnitude so let’s forgive them.
(I don’t dare make any comparison with Unicode Consortium and the Vogons here ;)
Or, comparing Encode to Babel Fish is completely appropriate -- if you can only
stick this into your ear :)
Surrogate pairs were born when the Unicode Consortium finally admitted that 16 bits
were not big enough to hold all the world’s character repertoires. But they
already made UCS-2 16-bit. What do we do?
Back then, the range 0xD800-0xDFFF was not allocated. Let’s split that range in
half and use the first half to represent the "upper half of a character" and the
second half to represent the "lower half of a character". That way, you can repre-
sent 1024 * 1024 = 1048576 more characters. Now we can store character ranges up
to \x{10ffff} even with 16-bit encodings. This pair of half-character is now
called a surrogate pair and UTF-16 is the name of the encoding that embraces them.
Here is a formula to ensurrogate a Unicode character \x{10000} and above;
$hi = ($uni - 0x10000) / 0x400 + 0xD800;
$lo = ($uni - 0x10000) % 0x400 + 0xDC00;
And to desurrogate;
$uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
Note this move has made \x{D800}-\x{DFFF} into a forbidden zone but perl does not
prohibit the use of characters within this range. To perl, every one of
\x{0000_0000} up to \x{ffff_ffff} (*) is a character.
(*) or \x{ffff_ffff_ffff_ffff} if your perl is compiled with 64-bit
integer support!
Error Checking
Unlike most encodings which accept various ways to handle errors, Unicode encodings
simply croaks.
% perl -MEncode -e ’$_ = "\xfe\xff\xd8\xd9\xda\xdb\0\n"’ \
-e ’Encode::from_to($_, "utf16","shift_jis", 0); print’
UTF-16:Malformed LO surrogate d8d9 at /path/to/Encode.pm line 184.
% perl -MEncode -e ’$a = "BOM missing"’ \
-e ’ Encode::from_to($a, "utf16", "shift_jis", 0); print’
UTF-16:Unrecognised BOM 424f at /path/to/Encode.pm line 184.
Unlike other encodings where mappings are not one-to-one against Unicode, UTFs are
supposed to map 100% against one another. So Encode is more strict on UTFs.
Consider that "division by zero" of Encode :)
SEE ALSO
Encode, Encode::Unicode::UTF7, <http://www.unicode.org/glossary/>, <http://www.uni-
code.org/unicode/faq/utf_bom.html>,
RFC 2781 <http://rfc.net/rfc2781.html>,
The whole Unicode standard <http://www.unicode.org/unicode/uni2book/u2.html>
Ch. 15, pp. 403 of "Programming Perl (3rd Edition)" by Larry Wall, Tom Chris-
tiansen, Jon Orwant; O’Reilly & Associates; ISBN 0-596-00027-8
perl v5.8.6 2001-09-21 Encode::Unicode(3pm)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
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